Facsimile machines employ a so-called fall-back strategy to reduce the speed of their associated modems whenever they encounter transmission impairments which degrade the page quality of received information (i.e. a facsimile page containing printed information, such as text and/or an image). Typically, the fall-back strategy is based solely on counting the number of scan lines on a page that contain errors due to, for example, transmission noise occurring on the associated communication facility. Thus, if the number of scan line errors on a received page happens to exceed a predetermined value--illustratively a value of 20--, then the receiving machine classifies the received page as being unacceptable. As a result thereof, the receiving and sending machines may renegotiate with one another to reduce the transmission rate as a way of hopefully improving the quality of succeeding pages. For example, if two facsimile machines are communicating with one another at a transmission rate of 9600 bps, they may elect to fall back to 7200 bps in the event that a received facsimile page is found to be unacceptable. Moreover, the machines may thereafter fall back to 4800 bps if the quality of the next received page is also found to be unacceptable as a result of the number of scan line errors exceeding the predetermined threshold.
I have recognized that as a result of using the same threshold for each transmission rate, prior facsimile machines are more apt to continue at the current rate of transmission, despite the fact that they have received pages with unacceptable quality. This is because many prior facsimile machines employ threshold values that are overly permissive of scan line errors in some applications. Consequently, the transmission speed may not be reduced in cases where the communication facility quality is inadequate for the current speed. Subsequent received pages in this case may also have unacceptable quality, where use of a lower speed may have produced satisfactory pages. Furthermore, when the same threshold is used for each transmission rate but the threshold values are overly critical of scan line errors, the facsimile machines are more apt to fall-back in speed once they have done so during a given facsimile transmission. In this case, the transmission rate may rapidly decrease to, for example, 2400 bps, thereby causing the duration, and hence, the cost of facsimile transmission to increase.
I have also recognized that such prior facsimile machines treat all scan line errors the same. That is, they do not distinguish between errors occurring in scan lines randomly disposed on a facsimile page and errors occurring in each of a series of contiguous lines. As a result, prior facsimiles machine are likely to conclude that the quality of a received page is acceptable even though the page may contain one or more of such series. However, in such an instance, the user of such machines may reach an opposite conclusion.